Axial flow gas turbine engines conventionally comprise a plurality of rotor discs, each of which carries an annular array of radially extending aerofoil blades on its periphery. One such disc carries the fan aerofoil blades to constitute an assembly which is positioned at the front of a ducted fan gas turbine engine.
The fan rotor assembly of a typical ducted fan gas turbine engine is large and therefore can be heavy. One way in which weight can be saved is to raise the hub to tip ratio of the assembly, that is the ratio of the diameter of the disc or hub to the diameter defined by the tips of the fan blades carried by the disc. This is achieved by increasing the diameter of the disc rim. However, this can only be done if the disc can be machined into a stress and weight efficient shape. Typically the disc is made as a single piece forging and the constraints imposed by its construction frequently prevent the disc being of the most efficient shape.